1. Field of the Invention
The invention concerns an illumination system for wavelengths ≦193 nm, i.e., VUV and EUV-lithography with a plurality of light sources, for example, as well as a mirror or lens device for producing secondary light sources, comprising several mirrors or lenses, divided into raster elements.
2. Description of the Prior Art
In order to allow even further reduction in the structural width of electronic components, especially to the submicron range, it is necessary to reduce the wavelength of the light used in microlithography.
For wavelengths smaller than 193 nm, lithography with weak x-rays or so-called EUV-lithography is discussed.
A suitable illumination system for EUV-lithography should homogeneously or uniformly illuminate, with as few reflections as possible, a pregiven field for EUV-lithography, especially the annular field of an objective lens, under lithography requirements. Furthermore the pupil of the objective lens should be illuminated up to a particular degree of filling σ, independently of the field, and the exit pupil of the illumination system should be situated in the entrance pupil of the objective lens.
Regarding the basic layout of EUV-illumination systems, we refer to the applicant's pending applications EP 99 106348.8, submitted on Mar. 2, 1999, entitled “Illumination system, especially for EUV-lithography”, U.S. Ser. No. 09/305,017, submitted on May 4, 1999 entitled “Illumination system particularly for EUV-lithography”, and PCT/EP 99/02999, submitted on May 4, 1999, entitled “Illumination system, especially for EUV-lithography”, whose disclosure contents are incorporated in their entirety in the present application.
The following are discussed herein as light sources for EUV-illumination systems:
laser plasma sources
pinch plasma sources
synchrotron radiation sources
Since light is emitted from these light sources, they are also examples of primary light sources.
In the case of laser plasma sources, an intensive laser beam is focused onto a target (solid, gas jet, droplet). The target is heated so strongly by the excitation that a plasma is formed. This emits EUV-radiation.
Typical laser plasma sources have a spherical beam, i.e., a radiation angle of 4 π, as well as a diameter of 50 μm to 200 μm.
In pinch plasma sources, the plasma is produced by means of electrical excitation.
Pinch plasma sources can be described as volume radiators (D=1.00 mm), which emit in 4 π, whereby the beam characteristic is dictated by the source geometry.
In the case of synchrotron radiation sources, one can distinguish three types of sources at present:                bending magnets        wigglers        undulators        
In bending magnet sources, the electrons are deflected by a bending magnet and emit photon radiation.
Wiggler sources comprise a so-called wiggler for deflection of the electron or an electron beam, and this wiggler comprises a multiple number of alternating polarized pairs of magnets arranged in rows. If an electron passes through a wiggler, it is subjected to a periodic, vertical magnetic field and the electron oscillates in the horizontal plane. Wigglers are also characterized by the fact that no coherency effects occur. The synchrotron radiation produced by a wiggler is similar to a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux that is intensified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In case of undulator sources, the electrons in the undulator are subjected to a magnetic field of shorter period and smaller magnetic field of the deflection poles than in the case of a wiggler, so that interference effects occur in the synchrotron radiation. The synchrotron radiation has a discontinuous spectrum based on the interference effects and emits both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
It is critical for an EUV-illumination system to provide a sufficiently high Lagrange optical invariant or etendu. The Lagrange optical invariant of a system is defined as the product of the illuminated surface times the square of the aperture.
If the numerical aperture in the plane of the wafer is in the range NAwafer=0.1-0.25, then in the case of 4:1 systems, a numerical aperture in the reticle plane of NAreticle=0.025-0.0625 is needed. If the illumination system is supposed to illuminate this aperture homogeneously and independent from the field up to a filling degree of σ=0.6, for example, the EUV-source must have the following 2-dim Lagrange optical invariant or etendu: (LC).LCill.=σ2LCObj=0.149 mm2−0.928 mm2
The Lagrange optical invariant LC, is generally defined as follows for the lithography system described herein:
LCill.=σ2x·y·NA2=σ2 A·NA2, wherein A is the illuminated area. The area comprises 110 mm×6 mm, for example, in the reticle plane.
The Etendu of a laser plasma source is defined as the product of the illuminated surface of an imaginary unit sphere around the source and the square of the Numerical Aperture at which each field point of the imaginary unit source sees the spherical source.LC=A·NA2ALPQ=2π[cos(θ1)−cos(θ2)]×(Rsphere)2, with Rsphere=1 mmNA≈rLPQ/Rsphere=0.100where θ1 is the minimum beam angle with respect to the optical axis and θ2 is the maximum beam angle with respect to the optical axisLCLPQ=2π[cos(θ1)−cos(θ2)]·r2LPQ
With the typical source parameters:    1. rLPQ=0.1 mm, θ1=0°, θ2=90° yields: LCLPQ=0.063 mm2. This corresponds to 27% of the required value of the Lagrange optical invariant LCill of, for example, 0.236 mm2.    2. rLPQ=0.025 mm, θ10°, θ2=90° yields: LCLPQ=0.0039 mm2. This corresponds to 1.7% of the required value of the Lagrange optical invariant of, for example, LCill=0.236 mm2.
The Lagrange optical invariant LCPinch of a pinch plasma source with a diameter of 1 mm, Ω=0.3 sr, for example, is:
 LCPinch=A·NA2=(π·1 mm2/4)·0.30532=0.073 mm2.
Thus, the pinch plasma source provides 31% of the required value of the Lagrange optical invariant of, for example, LCill=0.236 mm2.
The Lagrange optical invariant or Etendu for the undulator source can be estimated by a simplified model assuming a homogeneous two-dimensional radiator with diameter                               ∅          =                                    1.0              ⁢                                                           ⁢              mm              ⁢                                                           ⁢              and              ⁢                                                           ⁢              aperture              ⁢                                                           ⁢                              NA                Und                                      =                          0.001              ⁢                                                           ⁢              with                                                                                LC            Und                    =                      A            ·                          NA              2                                                                                                                                A                  Und                                =                                  π                  ·                                                            (                                              ∅                        /                        2                                            )                                        2                                                                                                                          =                                  0.785                  ⁢                                                                           ⁢                                      mm                    2                                                                                                                                NA            und                    =          0.001                                    as                                                  LC            Und                    =                                    A              ·                              NA                2                                      =                                          0.00000079                ⁢                                                                   ⁢                                  mm                  2                                            =                                                7.9                  ⁢                                                                           ⁢                  e                                -                                  07                  ⁢                                                                           ⁢                                      mm                    2                                                                                            ⁢           .
As can be seen from this rough calculation the Etendu or Lagrange optical invariant of the undulator source is much too small in comparison to the required value of the Lagrange optical invariant.
To increase the Lagrange optical invariant, an illumination system comprising a synchrotron radiation source known from U.S. Pat. No. 5,512,759, comprises a condenser system with a plurality of collecting mirrors, which collect the radiation emitted by the synchrotron radiation source and form it to an annular light beam that corresponds to the annular field being illuminated. By this, the annular field is illuminated very uniformly. The synchrotron radiation source has a beam divergence >100 mrad in the plane of radiation.
U.S. Pat. No. 5,439,781 shows an illumination system with a synchrotron radiation source, in which the Lagrange optical invariant, is adjusted by means of a scattering plate in the entrance pupil of the objective lens, wherein the scattering plate can comprise a plurality of pyramidal structures. Also, in U.S. Pat. No. 5,439,781, the synchrotron radiation source has a beam divergence >100 mrad. The synchrotron radiation according to U.S. Pat. No. 5,439,781 is also focused, for example, by means of a collector mirror.
The disclosure contents of both of the above-mentioned documents                U.S. Pat. No. 5,512,759        U.S. Pat. No. 5,439,781are incorporated into the disclosure contents of the present application by reference.        